New Delhi, Nov 3: Delhi's air quality slipped into the very poor category for the first time this season on Monday, as a thick layer of smog settled over the national capital and pollution levels surged after a brief improvement brought by last week's rain.
The Central Pollution Control Board (CPCB) recorded an Air Quality Index (AQI) of 307 at 7.05 am on Monday, November 2. By 8 am the reading had risen to 310. Out of 58 monitoring stations reporting data in the city, 36 recorded very poor air quality, while Jahangirpuri entered the severe category with an AQI of 417.
Five stations recorded moderate air quality and the remainder reported poor air. The CPCB classifies an AQI between 301 and 400 as very poor, and anything above 400 as severe.
Fine particulate matter, known as PM2.5, was the dominant pollutant at 51 of the 58 reporting stations on Monday morning, with PM10 the main pollutant at the remaining seven. PM2.5 particles are small enough to penetrate deep into the lungs and enter the bloodstream, which is why doctors pay close attention to them during Delhi's winter pollution season.
They are also the pollutant most closely linked to farm fires, vehicle exhaust, dust and other combustion sources that combine in the capital's winter air.
The deterioration was sharp, and it followed a strikingly clean spell. On Friday, after heavy rainfall washed pollutants out of the atmosphere, Delhi's AQI had dropped to 60, in the satisfactory category, giving the city its cleanest October air reading in four years. On Saturday the average was 117, in the moderate category.
By Sunday it had risen to 261, poor, and by Monday morning it had crossed into very poor. Delhi last recorded a very poor daily average on February 11, when the reading was 305, so Monday's figure marked a return to winter pollution conditions after months of comparatively better air.
Behind the headline number lies a story about timing, and about how the way pollution is measured can miss a large part of what is actually happening. According to an analysis of the episode reported by Hindustan Times, smoke from farm fires in Punjab and Haryana drove the rise, but most of those fires were lit in the evening, after the NASA satellite whose count is the standard measure of stubble burning had already passed over the region.
The standard satellite, Suomi NPP, passes over the region once a day at around 1.30 pm. Fires lit after that pass are not counted in its daily tally. Geostationary satellite data, which monitors thermal anomalies continuously through the day, tells a different story.
India's INSAT-3DS satellite recorded that over 90 per cent of intense crop fires in Punjab and Haryana were lit after 3 pm, and its evening fire counts climbed rapidly from 17 on October 29 to 90 on November 1. Other satellite observations have pointed in the same direction: data from Meteosat showed that around 98 per cent of fire heat detected in 2025 occurred after 3 pm, compared with 94 per cent in 2024.
In other words, the fires have shifted later in the day, and the afternoon satellite snapshot captures a shrinking share of them.
NASA satellite observations still detected 1,394 farm fires across Punjab and Haryana between October 1 and November 1, with Punjab accounting for 1,249 detections and Haryana for 145, including 171 fires recorded on November 1 alone. That total was below the 1,861 fires detected during the corresponding period last year.
But experts caution that the year-on-year comparison is misleading if the timing shift is ignored, because a growing share of burning now escapes detection altogether. The real number of fires, on this reading, is likely to be higher than the satellite count suggests in both years, and the apparent improvement may be smaller than it looks.
Meteorology then did the rest. Upper-level transport winds at 925 hectopascals blew directly from the northwest over Punjab and Haryana towards Delhi on October 31 and November 1, carrying dense smoke plumes into the National Capital Region. With calm conditions and northwesterly winds, pollutants remained in the atmosphere for longer instead of dispersing.
The analysis reported that during the overnight period of November 1, from 10 pm to 6 am on Monday, fine particulate matter averaged 194 micrograms per cubic metre, more than three times India's safe 24-hour limit of 60 micrograms per cubic metre.
The timing could hardly be worse. The deterioration comes just a week before Diwali, when firecracker emissions typically add to the pollution burden, and meteorologists warn that no favourable change in weather is expected over the next seven to ten days.
Meteorologist Navdeep Dahiya warned that conditions were becoming favourable for a sustained build-up of smog across Delhi-NCR and the northern plains, noting that many sensors across the region were reporting AQI readings above 300, with some moving towards the 400 to 500 range. The Air Quality Early Warning System has forecast that Delhi's air quality could deteriorate further into the very poor category by November 4.
For residents, the advice during such episodes is familiar but worth repeating: limit outdoor exertion, especially in the early morning when pollution peaks, keep windows closed where possible, use masks rated for fine particles when stepping out, and take extra care for children, the elderly and people with asthma or heart conditions.
Schools, offices and hospitals all feel the effects in different ways, from cancelled outdoor activities to a rise in respiratory complaints, and doctors routinely report a surge in patients during the first severe smog episodes of the season.
The deeper problem, however, is structural. Delhi's winter pollution is not caused by any single source. It is the combination of local emissions from vehicles, construction dust, industry and waste burning, regional emissions from farm fires in neighbouring states, and weather conditions that trap pollutants close to the ground.
Stubble burning has received the most public attention, partly because satellite images make it visible and partly because it is seasonal and, in principle, preventable. But the evening shift in burning times shows that enforcement and measurement must adapt together: a system designed around an afternoon satellite pass will systematically undercount fires that are deliberately lit after it.
Solutions discussed over the years include giving farmers viable alternatives to burning paddy stubble, such as machines that manage crop residue in the field, financial support for residue management, and stronger coordination between Punjab, Haryana and Delhi so that burning, wind patterns and air quality forecasts are considered together rather than state by state. None of these is quick, and none works on its own.
What Monday's episode demonstrates is that without accurate, all-day measurement, even judging whether progress is being made becomes difficult.
Delhi has been here before, and the pattern is depressingly familiar: a spell of rain clears the air, farm fires increase, winds turn northwesterly, and the AQI climbs through poor into very poor and sometimes severe. What is new in this episode is the clarity of the measurement gap. When more than nine out of ten intense fires are lit after the satellite has passed, the official fire count is not just incomplete, it risks giving a false sense of improvement.
Closing that gap, by using geostationary data alongside the afternoon pass, would give policymakers, farmers and residents a truer picture of what is being burned, when, and where the smoke is going.
Until then, Delhi faces a difficult week. With Diwali approaching, unfavourable winds forecast and no weather system expected to clear the air for seven to ten days, the capital's residents should expect the smog to persist and possibly worsen. Monday's crossing of the 300 mark was the season's first warning.
How the city, and the states whose fires feed its air, respond to that warning will determine whether it remains a warning or becomes the start of another long, grey winter.
(Reporting based on Hindustan Times and Oneindia.)